The present invention generally pertains to the manner of displaying video signals and is particularly directed to the progressive scan display of video signals derived from a film having successive image frames, such as motion picture film.
In an NTSC television system, video signal displays typically are provided by interlacing video fields. Odd and even video fields are alternatively transitted at 60 fields per second, and upon receipt, the odd and even fields are displayed in an interlaced format to provide video frames at 30 frames per second.
The concept of interlaced-scanning displays was devised as a compromise between vertical resolution, temporal resolution, and transmission bandwidth. In theory, if the bandwidth is fixed, then the interlaced display is capable of achieving either twice the vertical resolution or twice the temporal resolution of the corresponding progressive-scan display. Unfortunately, there are disadvantages associated with interlaced systems. Much of the improvement is lost when both motion and high vertical frequencies occur simultaneously. Even for still pictures, the vertical resolution is limited by what is known as the Kell Factor. It has been determined that the subjective improvement in resolution for an interlaced television system having twice the scan lines of a progressive scan system does not exceed 25 to 30%. Oher penalties include interline flicker and line crawl--two disturbing defects associated with interlaced systems. Both become visible when high vertical frequencies are present.
Almost all existing algorithms for achieving progressive scan display from an interlaced transmission format do so by estimating the missing lines in each field. The simplest estimator would either repeat an adjacent line or average the two neighboring lines in the same field. Unfortunately, this reduces vertical resolution and defeats the original purpose of interlace. A better solution is to fill in the missing lines by replicating the same line or averaging the same two lines in the previous and following fields. Although this reduces temporal resolution, it achieves the correct result at the times when the viewer is most sensitive to defects--when stationary images are displayed. Unfortunately, it is more costly to implement since a frame store or field store is required.
More effective progressive scan algorithms use a combination of both vertical averaging and temporal averaging. Through the use of a simple motion detector, a decision is made to perform temporal averaging if the image or region is believed to be stationary, and vertical averaging otherwise. In this case, the result is correct when the image is stationary but a loss in vertical resolution still occurs in the presence of motion. Often used arguments claiming reduced viewer sensitivity to moving spatial details can be disputed due to the very high likelihood of visual tracking by the viewer. As a result, the sudden loss in sharpness that occurs during the transition from stationary to moving detail can be quite noticeable. Other disadvantages are the susceptibility of the motion detector to errors and the requirement of a frame store at the receiver.
The next improvement in progressive scan implementations is often accomplished through the use of an error channel. For instance, Tsinberg, "Philips NTSC-Compatible Two-Channel Television System", Third International Colloquium on Advanced Television Systems: HDTV87, Oct. 4-8, 1987, Ottawa, Canada describes the use of a time-compressed line-difference signal to improve the estimates obtained by vertical averaging; while Isnardi et al., "A Single Channel, NTSC Compatible Widescreen EDTV System", Third International Colloquium on Advanced Television Systems: HDTV87, Oct. 4-8, 1987, Ottawa, Canada, describe the use of a low-bandwidth vertical-temporal "helper" signal to improve the estimates derived by temporal averaging. Hence, these recent methods all used interpolation and then sought to eliminate the accompanying artifacts by sending additional information.
A different approach was proposed by Lucas, "B-MAC and HDTV - How Does It Fit?" Third International Colloquium on Advanced Television Systems: HDTV87, Oct. 4-8, 1987, Ottawa, Canada, wherein the information contained in two vertically-adjacent scan lines of a 525 line progressive scan system was multiplexed into a single scan line of a 525 line interlaced system. At the receiver, the order of information is restored and then displayed in progressive scan format. Unfortunately, if the horizontal bandwidth is fixed, only half of the original video samples can be sent. In this case, a loss in diagonal resolution occurs. Some degradation can also be expected on existing receivers.